Diagnostic imaging devices include roentgen devices for X-ray photographs, X-ray CT devices, PET devices, MRI devices and diagnostic ultrasonic devices, each of which has good points and bad points in terms of the cost for installing the device, the ability to diagnose a certain body area, the time for examination, whether or not there is a problem of exposure to radiation, and whether or not a chemical needs to be injected, and thus, they are used for different purposes of examination.
From among these diagnostic imaging devices, diagnostic ultrasonic devices and MRI devices are excellent with regards to safety because there are no problems of exposure to radiation. In addition, diagnostic ultrasonic devices are superior to MRI devices that also do not use radiation in the points that the cost of the device is low and the device is compact, which makes it portable.
Visceral obesity is cited as one risk factor of diseases from poor living habits, such as myocardial infarction, cerebral infarction, diabetes and fatty liver, and thus, those who have a high risk of getting a disease from poor living habits or obesity are diagnosed as having metabolic syndrome so that measures for preventing a disease can be taken from the point of view of preventive medicine. In order to diagnose whether a person has metabolic syndrome, it is necessary to check the amount of visceral fat.
In the case where a lump is found during mammary cancer screening, it is necessary to examine whether the lump is merely fat, which is a benign tumor, or a malignant tumor. In such a case, if whether the lump is made of fatty tissue can be easily detected, it can be judged to be benign or malignant using other references.
As described above, in some cases it is necessary to examine whether or not some tissues in a region of interest (ROI) within a living body are fatty tissue. In such a case, fatty tissue can be detected through image diagnosis using an X-ray CT device. However, the use of an X-ray CT device gives rise to the problem of exposing a subject to radiation.
Therefore, an examination for fatty tissue using ultrasonic wave tomography has been proposed as a safe imaging diagnostic method that does not cause a problem of exposure to radiation (see Non-Patent Document 1).
The velocity of the ultrasonic waves that propagate through muscles and internal organs (intestines, kidneys) is similar to the velocity of sound that propagates through water at a temperature of 37° C., and thus is generally 1500 m/sec or faster. In contrast, the velocity of the ultrasonic waves that propagate through fatty tissue is 1500 m/sec or slower, and therefore, the distribution of fat in the portion of a subject to be measured (abdominal region, for example) can be obtained from the data on the velocity of the ultrasonic waves in the case where it is possible to measure the velocity of the ultrasonic waves travelling through the tissues that form the abdominal region.
According to the ultrasonic wave tomography disclosed in the above document, the difference in the velocity of the sound that travels through different substances is used to estimate the amount of visceral fat by means of transmission measurement.
Meanwhile, a method for taking a tomographic image of the distribution of a change in the velocity of the ultrasonic waves in a target region through irradiation with light (tomographic image of the distribution of light absorption) where a conventional reflection type diagnostic ultrasonic device is used, a mechanism for irradiating a target region with light is provided, and a change in the velocity of an ultrasonic wave echo signal after irradiation with light, as compared to the time before irradiation with light, is calculated (see Patent Document 1).
The tomographic image of this change in the velocity of the ultrasonic waves (tomographic image of the distribution of light absorption) shows a change in the temperature in the target region due to the absorption of light for irradiation. That is to say, when a living body is irradiated with light, the distribution of the temperature within the living body depends on the light absorbing properties of the respective portions. The velocity of the ultrasonic waves that propagate through the living body changes depending on the temperature, and therefore, a change in the velocity of the ultrasonic wave echo signal after irradiation with light as compared to before irradiation with light can be found for each portion so that a tomographic image is gained, and thus, the distribution of the change in the velocity of the ultrasonic waves, the distribution of a change in the temperature or the distribution of light absorption can be displayed as tomographic images.
Thus, fatty tissue displaying devices with which fatty tissue of a subject can be displayed by means of a reflective type diagnostic ultrasonic device using the properties of the ultrasonic waves that pass through the fatty tissue and muscle/internal organ tissue and of which the velocity changes depending on the temperature have been proposed (see Patent Document 2).
In general, changes in the velocity of the ultrasonic waves depending on the temperature are compared as follows:
Water: +2 m/sec·° C.
Fat: −4 m/sec·° C.
That is to say, the velocity of the ultrasonic waves that travel through muscle or internal organs (intestines, kidneys) that include a large amount of water increases as the temperature rises, while the velocity of the ultrasonic waves that travel through the fat portions lowers, and thus, the polarity in the change of the velocity of the ultrasonic waves reverses. Therefore, an ultrasonic wave echo signal that has been received from the target region of a subject when not irradiated with near infrared rays and an ultrasonic wave echo signal that has been received from the target region after being irradiated with near infrared rays are received by an ultrasonic imaging probe fabricated with array transducers. Subsequently, data on the change in the velocity of the ultrasonic waves after irradiation with light in the target region is calculated on the basis of the ultrasonic wave echo signal before irradiation with light and the ultrasonic wave echo signal after irradiation with light. Then, the regions where a negative change in the velocity of the ultrasonic waves is exhibited after irradiation with light are identified as fatty regions from the calculated data on the change in the velocity of the ultrasonic waves.
FIGS. 13(a) to 13(c) are diagrams showing an example of tomographic images of fatty tissue described in Patent Document 2, where FIG. 13(a) is a diagram for illustrating a phantom that is a target object, FIG. 13(b) is a B mode image thereof, and FIG. 13(c) is an image showing the change in the velocity of the ultrasonic waves.
The main body of the phantom is made of an agar including intralipid that is a light scattering material and a piece of beef tallow including a carbon powder is buried in a portion of the phantom so as to be a fatty region. The carbon powder is provided in order to efficiently heat the piece of beef tallow when irradiated with light and functions as a contrast medium. A piece of agar including a carbon powder having approximately the same size as the piece of beef tallow is also buried as a reference for comparison.
This sample was irradiated with a laser beam with a wavelength of 809 nm for approximately 15 seconds (0.1 W/cm2) so that data on a change in the velocity of the ultrasonic waves could be obtained. The regions where the velocity of the ultrasonic waves exhibited a negative change after irradiation with light are indicated by blue, and the other regions are indicated by red.
As a result, the region that includes the fatty region is indicated by blue so that the position can be clarified in the image of a change in the velocity of the ultrasonic waves in FIG. 13(c), though this can barely be distinguished in the B mode image in FIG. 13(b).